Method of fabricating a dark heater

ABSTRACT

A refractory metal heater wire is first coated in conventional fashion with a first coating of insulating material, e.g., aluminum oxide. The coating is then dried in air at a temperature below which oxidation of the heater wire occurs for a time sufficient to completely remove all solvents from the first coating. Then, a second coating of a darkening material is applied over the first coating. The complete removal of solvents from the first layer prior to the second layer application step reduces penetration of the material of the second layer into the first layer.

United States Patent [1 1 Hale et al.

[ METHOD OF FABRICATING A DARK HEATER [75] Inventors: John Richard Hale,Lancaster, Pa.;

George Irvin Merritt, Clifton, NJ.

[52] US. Cl 117/217, ll7/227, 313/345 [51] Int. Cl B44d l/l8 [58] Fieldof Search 117/217, 29, 31, 227

[56] References Cited UNITED STATES PATENTS 6/1967 Scheible 117/2179/1968 Feinleib 117/217 [451 Apr. 30, 1974 Primary Examiner-Cameron K.Weiffenbach Attorney, Agent, or Firm-G. l-l. Bruestle; L. Greenspan [57]ABSTRACT A refractory metal heater wire is first coated in conventionalfashion with a first coating of insulating material, e.g., aluminumoxide. The coating is then dried in air at a temperature below whichoxidation of the heater wire occurs for a time sufiicient to completelyremove all solvents from the first coating. Then, a second coating of adarkening material is applied over the first coating. The completeremoval of solvents from the first layer prior to the second layerapplication step reduces penetration of the material of the second layerinto the first layer.

3 Claims, N0 Drawings METHOD OF FABRICATING A DARK HEATER BACKGROUND OFTHE INVENTION This invention relates to electron discharge tubes, andparticularly to a method of fabricating dark insulated heaters for suchtubes.

In various types of electron tubes having indirectly heated cathodes, itis the practice to use dark heaters to heat the cathode to electronemitting temperatures. Such heaters comprise a core wire of a refractorymetal, such as tungsten, a first wire covering layer of an insulatingmaterial, such as aluminum oxide, and an outer dark coating such as aparticulate mixture of tungsten and aluminum oxide. A purpose of thefirst coating is to provide insulation between the heater wire and thecathode, and a purpose of the outer coating is to increase the thermalemissivity of the heater, thereby lowering the temperature at which theheater need operate to heat the cathode to'its operating temperature.The greater the proportion of tungsten in the outer coating, the darkerthe heater.

While the use of insulated heaters having an outer coating consistingentirely of tungsten has been suggested in the past (see, for example,US. Pat. No. 3,195,004 issued to Hassett on July 13, 1965), the use ofsuch heaters has generally not been practical. One reason for this isthat, in the past, as the percentage of tungsten in the outer coatingwas increased, the amount of current leakage between the heater and thecathode increased. The cause of such leakage is that particles oftungsten from the outer coating penetrate into the aluminum oxideundercoating and provide leakage paths for current through theundercoating. This is generally undesirable, and the practice in thepast has beento limit the ratio of tungsten to aluminum oxide in theouter coating to some upper limit, e.g., in the orderof 40 percent.

A technique has recently been suggested (as described in co-pending US.Pat. application Ser. No. 242,240 filed Apr. 7, 1972 for John I-Iale)for fabricating heaters having outer coatings with a large percentage,ashigh as 100 percent, of tungsten while avoiding high levels of.leakage through the heater. Briefly, the technique comprises using acoating bath for the outer coating which isrheologically stable, i.e.,which has a low settling rate of the particles therein. This results ina high viscosity bath whereby penetration of the bath into theundercoating is prevented.

In addition, to further prevent such penetration, it has, in the past,been thought necessary to render the undercoating as impervious to theovercoating bath as possible by means of a high temperature sinteringprocess prior to performing the overcoating process. Disadvantages ofthe sintering process, however, are that expensive apparatus isgenerally required, the process tends to be time consuming andexpensive, and, owing to the high temperatures involved, variousfixtures, such as clips in which the heaters are mounted, have a shortlife requiring frequent maintenance and replacement. Also, to preventoxidation of the heater wire, the sintering is done in a protectiveatmosphere, which adds further cost and complexity to the process.

DETAILED DESCRIPTION OF THE INVENTION One methodgenerally known forapplying various coatings to heater structures is known as dip" coating.This method involves dipping the heater to be coated into and out of abath containing an organic solvent and a suspension of the particles tobe coated on the heater, the particles adhering to the heater as theemerging heater breaks through the surface of the bath. If the heaterstructure is in the form of a continuous wire or the like which is drawnthrough the bath, the process is known as a drag coating process. Thethickness of the coating is a function of the specific gravity andviscosity of the bath.

As previously noted, a problem in the past using dark heaters havinglarge percentages of tungsten in the outer coating is that the tubesusing the heaters generally have unacceptably high levels of heater tocathode leakage. This occurs because of penetration of particles oftungsten from the outer coating into the aluminum oxide undercoating.

We have discovered that probably the principal cause of penetration ofthe tungsten particles into the undercoating is the use of outer coatingprocesses in which the viscosity of the coating bath is allowed tobecome too low. In such case, as described in the abovecited co-pendingapplication, the highly fluid bath, including tungsten particlesconveyed therewith, penetrates into the undercoating.

Although it is generally desirable that the undercoating be renderedrelatively hard and impervious prior to the overcoating process, by ahigh temperature firing operation, for minimizing penetration of theovercoating bath into the undercoating, we have discovered that such ahigh temperature process is not necessary, and that penetration can beavoided even if the undercoating is in a non-sintered, relatively softand porous condition during the overcoating process.

What is necessary, we discovered, is that the undercoating besubstantially completely dry and free of all solvents used both in theundercoating process and in the usual rinsing step performed thereafter.That is, we discovered that the prevention of penetration of theovercoating bath into the undercoating is not so much a function of thehardness and imperviousness of the undercoating, as previously thought,but primarily a function of the degree of removal of the solvents fromthe undercoating. While not known for sure, it appears that the presenceof such solvents in the undercoating tends to dilute and reduce theviscosity of the overcoating bath which contacts the undercoating, thuspromoting penetration.

The prior art process of sintering the undercoating prior to theovercoating process also results in a complete drying and removal of thesolvent. The advantage of our discovery, that sintering of theundercoating is not necessary, is that a much lower temperature dryingoperation can be used, thereby allowing the use of simple apparatus, noreducing atmosphere, and a processing rate higher than was heretoforepossible.

In a specific embodiment of the invention, a heater wire of usualconfiguration is first coated by a conventional means with a layer ofaluminum oxide having a thickness in the order of 5 mils. The coatingoperation can comprise the known dipping, spraying, or cataphoreticprocesses. In such operations, as known, an organic solvent or vehiclefor the aluminum oxide is used which tends to remain with the aluminumoxide coating on the heater wire. Also, the aluminum oxide layer issomewhat soft and porous.

While it is not possible to give a quantitative definition of what ismeant by soft" and porous," the difference in softness and porosity ofan aluminum oxide layer before and after a high temperature firingoperation is well known to persons skilled in these arts. In general,the soft layer can be scraped off the heater wire in a paste-like form,whereas the sintered layer is quite brittle and crumbles upon beingscraped or bent.

Thereafter, in accordance with usual techniques, the coated heater isrinsed in a suitable solvent, such as methanol or acetone, to removeloosely adhered particles.

Then, in accordance with the instant invention, the heater is air driedat a temperature as high as possible for rapidly drying the heaterwithout causing significant oxidation of the heater wire. Thetemperature used, and the time required to thoroughly dry the heater,are functions of the configuration of the heater and the size and massof the heater. In general, however, to avoid oxidation of the heaterwire and thus avoid the need to use a protective atmosphere during thedrying operation, temperatures below 400 C are preferred.

Although the drying operation is performed at a temperature well belowthat used in the prior art sintering process (generally above l,600 C),some degree of hardening of the insulation coating occurs. This isdesirable for further reducing the possibility of bath penetration.

Various means for performing the drying process can be used. Forexample, heaters can be placed in a conventional air oven and heated.Alternatively, a jet of hot dry air can be blown over the heaters.Further still, a source of radiations, such as infrared radiations froma conventional source, can be directed onto the heaters. The latterheating means is preferred since it is simple, noiseless, and theradiations can be accurately focused onto the desired portions of theheaters, thereby avoiding heating of fixtures used to carry the heaters.

In one embodiment of the invention, in which is air dried a heaterhaving an undercoating of aluminum oxide of 5 mils thickness and weightof 5 miligrams, and a base wire of tungsten of 54 mm length and 3.5 milsdiameter an infrared source is used to heat the heater coating to atemperature of about 300 C for a time of l2 seconds.

After thorough drying, the coated heaters are provided with the outercoating of tungsten. Preferably, in order to avoid penetration of thetungsten into the undercoating, a coating bath is used in accordancewith the process described in the above-described co pendingapplication. For example, with a coating bath comprising (by weight) 40percent tungsten and 60 percent liquid vehicle, the tungsten having aparticle size of 0.5 to microns, with the average particle size beingabout 2 microns, the vehicle comprising 1.68 percent lQQQ secondnitrocellulose wet 30 percent by weight by alcohol and 98.5 percentbutyl acetate, the bath is ball milled for about 6 hours. This providesa rheologically stable bath suitable for coating the tungsten particlesonto the heater without bath penetration into the undercoating.

Other overcoating techniques, such as spraying or cataphoreticprocesses, can be used.

After the overcoating process, the heater is again rinsed, in a suitableorganic solvent, to remove loosely adherent particles, and the heater isfired at an elevated temperature, in the order of l,600 C to completethe heater.

While the invention has been described using specific materials, theinvention has utility in the fabrication of heaters using othermaterials. For example, the heater wire can be any refractory wire, suchas molybdenum, normally used in heaters.

The undercoating can be various known insulating refractory materialsnormally used in heaters, such as zirconium oxide, beryllium oxide, andthe mixture of chrome oxide and titanium oxide. These materials, andothers known to workers skilled in these arts, tend to be relativelysoft and porous when initially applied, and are normally later fired athigh temperatures. Thus, the above-described problems of the prior artprocesses exist with the use of such materials, and advantages areobtained using the inventive process described herein.

Also, the darkening outer coating can comprise various refractory, highthermal emissivity materials such as carbon, titanium, chromium, andmolybdenum. Again, avoidance of penetration of these materials into theunderlying coating is generally to be desired.

We claim:

1. A method of fabricating a dark heater comprising:

coating a tungsten wire with a relatively soft and porous first layer ofaluminum oxide containing a solvent,

heating said coated wire in air at a temperature below 400 C for a timesufficient to remove said solvent from said first layer,

coating said coated wire with a second layer of tungsten metal particleswhile said first layer is still relatively soft and porous and firingsaid coated wire at an elevated temperature to complete the heater.

2. A method of fabricating a dark heater comprising:

coating a refractory metal heater wire with a relatively soft and porousfirst layer of electrically insulating inorganic material containing asolvent,

heating said coated wire in air at a temperature below 400 C and belowwhich oxidation of said wire occurs for a time sufficient to remove saidsolvent from said first layer,

coating said coated heater wire while said first layer is stillrelatively soft and porous with a second layer containing refractorymetal particles of high thermal emissivity, and

firing said coated heater wire to sinter said coatings.

3. The method of claim 2 wherein said second layer is coated by aprocess in which a solvent is caused to contact said first layer.

2. A method of fabricating a dark heater comprising: coating arefractory metal heater wire with a relatively soft and porous firstlayer of electrically insulating inorganic material containing asolvent, heating said coated wire in air at a temperature below 400* Cand below which oxidation of said wire occurs for a time sufficient toremove said solvent from said first layer, coating said coated heaterwire while said first layer is still relatively soft and porous with asecond layer containing refractory metal particles of high thermalemissivity, and firing said coated heater wire to sinter said coatings.3. The method of claim 2 wherein said second layer is coated by aprocess in which a solvent is caused to contact said first layer.